Electric furnace having replaceable liner tube sections



Jan. 14, 1969 A. M. PlsANo ET AL. 3,422,205

ELECTRIC FURNACE HAVING REPLACEABLE LINER TUBE SECTIONS Sheet l of 5 Filed Sept. 29. 1966 INVENTORS .52D mmIwDm N M zoN wrlmmn.

L ANTHONY Ow vwNA-L M. PISANO JOHN K. BARNARD 42d ATTORNEY Jan. 14, 1969 A. M, PlsANo ET AL `3,422,205

ELECTRIC FURNACE HAVING REPLACEABLE LINER TUBE SECTIONS Sheet Filed Sept. 29, 1966 mZON ...OI

INVENTORS ANTHONY M. PISANO JHON K. BARNARD` ATTORNEY SheeiI of 5 mZON 023000 .mm1-.55.200

NNI NN- TNv L NN- A. M. PISANO ET AL INVENTORS ANTHONY M. PISANO JOHN K. BARNARD BY ATTORNEY Jan. 14, 1969 ELECTRIC FURNACE HAVING REPLACEABLE LINER TUBE SECTIONS Filed Sept. 29, 1966 omi Jan. 14, 1969 A. M. PlsANo ET Al. 3,422,205

ELECTRIC FURNACE HAVING REPLACEABLE LINER TUBE SECTIONS ANTHONY M. PISANO BY JOHN K. BARNARD ATTORNEY Jan. l14, 1969 A M. PISVANO ET Al. ELECTRIC FURNACE HAVING REPLACEABLE LINER TUBE SECTIONS Filed Sept. 29. 1966 Sheet O mma mm mpm NMA J .l B w Y NK a m m Aw F 2 w 2 M021/ AT TORN EY United States Patent O 3,422,205 ELECTRIC FURNACE HAVING REPLACEABLE LINER TUBE SECTIONS Anthony M. Pisano and John K. Barnard, Youngstown, N.Y., assignors to The Carborundum Company, Niagara Falls, N.Y., a corporation of Delaware Filed Sept. 29, 1966, Ser. No. 582,866 U.S. yCl. 13-20 11 Claims Int. Cl. H05b 3/6'2 ABSTRACT OF THE DISCLOSURE A horizontal tube electric furnace having a replaceable graphite liner tube formed of a series of hollow, open ended, liner tube sections which are detachably joined together. The liner tube carries work containing receptacles which are moved relative to said liner tube. Means are provided for advancing the liner tube sections axially through the furnace, either intermittently or continuously, with used sections being removed from the rear en-d of the composite liner tube while new sections are added to the forward end of said liner tube.

This invention relates to electric furnaces and, more particularly, to continuous, high temperature, horizontal tube furnaces.

Electric furnaces of the horizontal tube type are customarily characterized by having an elongated tube, commonly referred to as a liner tube in the art, and generally consisting of sections extending through a portion of the furnace. Normally the material to be furnaced is advanced through this liner tube in boats or capsules.

A major problem experienced with many iiring processes in horizontal tube furnaces is that solid deposits or contamination occurs in the liner tube beyond a tolerable level, and, after a period of time, far shorter than the normal useful life of the tube, further processing becomes increasingly diiicult and eventually impossible. As a result, the furnace has to be shut down and the liner tube replaced. When employing the furnace at elevated operating temperatures, the rebuilding operation becomes intolerable since the furnace may be operable for less than a week between shut-downs. The rebuilding operation includes cool-down time and heat-up time in addition to furnace breakdown and restoration. For the specific furnace -described below, total shutdown time is approximately four days including an 8-hour shift for four craftsrnen to rebuild the furnace. It is readily apparent that efficiency and production are seriously impaired.

The horizontal tube furnace of the present invention, as hereinafter described, provides a solution to the above problem by providing a series of detachably joined liner tube sections forming a substantially elongated replaceable liner tube and which are moved axially through the furnace. As the tube sections are advanced through the furnace, new sections are added at the entrance end of the furnace while used sections are removed from the remote or exit end without interfering with the continuous firing operation taking place.

Accordingly, it is an object of the present invention to provide a new and improved horizontal electric tube furnace.

It is another object of the present invention to provide a new and improved horizontal electric tube furnace having novel liner tube means.

It is still another object of the present invention to provide a new and improved horizontal electric tube furnace in which the liner tube is formed of detachably joined sections extending completely through the furnace.

It is a further object of the present invention to provide a new and improved horizontal electric tube furnace in which the liner tube is formed of detachably joined sections that are advanced axially through the furnace.

These and other objects of the invention will become more apparent upon consideration of the following detailed description thereof when taken in conjunction with the following drawings, in which:

FIGS. 1A, 1B, 1C, and 1D, taken together, constitute a longitudinal section of the horizontal tube furnace constructed in accordance with the principles of this invention;

FIG. 2 is a cross sectional view taken through the preheat zone on the plane of line 2-2 of FIG. 1A;

FIG. 3 is a cross sectional view taken through the hot zone on the plane of line 3--3 of FIG. 1B;

FIG. 4 is a cross sectional View taken through the cooling zone on the plane of line 4-4 of FIG. 1C; and

FIG. 5 is an end View of the heating element viewing such element at its forward end.

With reference to FIGS. 1A, 1B, 1C and 1D, it will be seen that a preferred embodiment of a horizontal tube furnace constructed in accordance with the principles of this invention comprises a Icasing consisting of a pusher unit, generally designated 10, a preheat zone section, generally designated 12, a hot zone section, generally indicated at 14, a controlled cooling zone section, indicated generally as `16, a cooling zone section, generally designated 18, and a water-cooled exit chamber section, generally indicated as 20. These sections are supported on a base having suitable pedestals or standards 22, the speciiic configuration of each standard being rdependent on the weight and stress of its associated section and will vary in accordance with the economics of design. Since the specilic design of the standards =forms no part of this invention, no further amplification or description is believed necessary.

Extending longitudinally through the furnace is an elongated graphite liner tube, generally designated as 24, formed of a plurality of hollow, open ended liner tube members or sections 26, the ends of which are rabbeted with outside and inside rabbets 28 and 30, respectively, whereby the sections can be detachably joined. Although the ends of the liner tube sections are preferably rabbeted, it should be appreciated that any suitable connection means may be employed for connecting the sections together within the purview of the present invention. Adjacent the exit end of the furnace are a plurality of relatively short liner tube sections 27 for a purpose to be hereinafter more fully explained, said short sections adapted to be detachably joined in the same manner as the longer liner tube sections 26 to form the composite liner tube 24. It will be seen that the forward end of the liner tube 24 generally protrudes beyond the forward end of the furnace, the forward end taken to be the entrance or left end of the furnace as seen in FIG. 1A.

The pusher unit 10 comprises an endless chain 32 trained about and driven by drive sprocket 34 and guided for movement about idler sprockets 36, 38 and 40. Drive sprocket 34 is mounted on a shaft `42 rotatably driven by a suitable motor 44 through means of a conventional gearing reduction mechanism 46. Rollers 48 of the chain 32 are adapted to engage bifurcated dogs S0 and be strattled thereby, said lugs rigidly secured to and dependent from pusher plate 52, which in turn is rigidly secured to a pusher block 54 for engaging and advancing the liner tube 24. Guides 56 are provided on pusher unit 10 for facilitating the movement of said liner tube.

The preheat zone section 12 comprises a casing or shell 60 of either a square -or rectangular cross section, formed preferably of steel, and lined with suitable refractory bricks 62 to define a furnace chamber 64. The bricks disposed nearest the longitudinal axis of the furnace chamber are preferably formed of a composition to withstand higher temperatures than the bricks located adjacent the shell 60. End walls 66 and 68 of the furnace chamber 64 are provided with circular openings 70 and 72 respectively, for receiving the liner tube 24 and the lower arcuate surface of said end walls are recessed to for-m shoulders 74 and 76, respectively, for accommodating an elongated graphite liner tube support slab 78 of a curved trough-like cross section conforming to the cylindrical outer Surface of the liner tube 24.

AResting on shoulders 88 and 90 of side walls 84 and 86, respectively, and on shoulders 92 and 94 of end wall 66 and 68, respectively, is an elongated graphite slab 96 forming the roof of the furnace chamber 64. Extending through the shell 60 and the side walls 84 and 86 transversely across the upper and lower portions of furnace chamber 64 are a plurality of elongated heating elements 98 suitably connected at their opposite ends to a suitable power source (not shown). Although a total of eight heating elements are illustrated, it should be appreciated that more or less than eight heating elements may be employed, as desired, within the purview of this invention.

Shell 60 is provided with a flange 100 at its rearward end and is provided with an integral cylindrical section 102 at its forward end having an inner surface complementary to the outer surface of the liner tube 24. A burner ring 104 surrounds the cylindrical section 102 at the forward end thereof to preclude any oxygen from entering into the furnace chamber.

The hot zone section 14 comprises a casing or shell 110, preferably formed of steel, and of either a square or rectangular cross section and is provided with a flange 112 at its forward end which is suitably rigidly secured to the flange of shell 60 as by means of welding, for eX- ample. Supported on the floor of shell is a tube centering block 116 for guiding and positioning the liner tube 24 within the shell 110.

Tube centering block 116 is formed with an indentation 118 on its forward face 120 to receive a three-phase graphite resistance element 122. A major portion of the heating element 122 intermediate the end thereof is reduced in cross section as shown at 134 in FIG. 1B to increase the resistance in this area. Such heating elements are conventional and consist of three sections, 124, 126, and 128, each having an arcuate cross section of approximately as illustrated in FIGS. 3 and 5 and are provided with ceramic inserts between the sections atthe forward ends thereof to maintain the sections separated and to prevent the collapse thereof. The bottommost of these inserts 130 has a T-shaped cross section and is provided with a wear-resistant surface 132 upon which the liner tube 24 is advanced. The other two inserts do not engage the liner tube and are used solely to prevent inward radial movement of the sections.

The forward face 120 of centering block 116 is provided with a second stepped portion or indentation 136 to receive an elongated, cylindrical, graphite, primary protection tube 138 for a purpose hereinafter explained. On the rearward face 140 of centering block 116, a stepped portion or indentation 142 is provided for receiving an elongated, cylindrical, graphite secondary protection tube 144 which extends from said rearward face 140 to an insulating support 146 having a shoulder 148 therein for accommodating the rearward end of the secondary protection tube 144. Disposed between the secondary protection tube 144 and the liner tube 24 is an elongated graphite liner tube support slab 150 having the same general cross section as support slab 78. Shell 110 is provided with a flange 154 at its rearward end adapted to be joined to the controlled cooling zone section 16.

Circumferentially disposed about the periphery of the heating element 122 adjacent the forward end thereof and extending radially outwardly to the shell 110 is an insulating support member 158. The interior space of the shell 110 surrounding the primary protection tube 138 and the secondary protection tube 144 between insulating support member 158 and insulating support 146 is filled with a particulate mass of thermal insulating material, such as carbon black by way of example. Thus, it will be seen that the primary protection tube 138 and the secondary protection tube 144 separate the carbon black from the heating element 122 and the liner tube 24, respectively, and either of these last mentioned elements may be replaced without removing the carbon black.

A terminal assembly 174 is suitably electrically connected to the forward portion of heating element 122 on the forward side of insulating support member 158 and suitably electrically connected at its other end to a suitable source of electric power in a conventional manner well known in the art. The terminals are cooled by means of conventional watercooled piping.

The controlled cooling zone section 16 comprises a casing or shell 180, preferably formed of steel, and of either a quare or rectangular cross section and is provided with a ange 182 at its forward end which is suitably rigidly secured to the flange 154 of shell 110 as by means of welding, for example. The controlled cooling zone section 16 is similar in construction to the preheat zone section 12 with the exception that it is a -mirror image thereof or, stating it another way, oriented l from that of the preheat zone section. Since the remainder of the structure is identical to that of the preheat zone section, no further detailed description is believed necessary, it being understood that the same reference characters will identify similar elements.

The rearward portion of the controlled cooling zone section 16 comprises a cylindrical casing or shell 200 rigidly secured to shell 180, as by means of welding, for example, and is provided with an annular flange 202 at its rearward end adapted to be joined to the cooling zone section 18. An elongated graphite liner tube support slab 204, having an inner arcuate surface (see FIG. 4) complementary to the outer surface of the liner tube 24 is provided for supporting the liner tube 24.

The cooling zone unit 18 is provided with a cylindrical casing or shell 210, preferably of steel, having a flange 212 adapted to be suitably joined to the flange 202 of shell 200. The inner surface 214 of shell 210 is circular to accommodate the liner tube 24 which rides directly on such inner surface. A water jacket 216 encloses shell 210 for permitting the circulation of a coolant therethrough to transfer heat from the liner tube 24 to the circulating coolant. Shell 210 is provided with a flange 218 at its rearward end adapted to be joined to the flange 220 of the water cooled exit chamber 20.

The water cooled exit chamber 20 is similar in construction to the cooling zone section 18 except for the provision of a pivotable door 222 at its rearward end. Similar elements will be identified by identical reference characters. -Door 222 is rigidly secured to a lug 224, which in turn is rigidly secured to an L-shaped handle 226 pivotably mounted on shaft 228 disposed in a bracket 230 which is suitably secured to shell 210. Normally the door is gravitationally held in the closed position by its own weight and is opened by applying force to handle 226 in a counterclockwise direction.

Preferably, but not limited thereto, the apparatus of this invention may be employed in a furnacing or siliconizing process to form an impermeable, dense, selfbonded silicon carbide tinished product. The siliconizing step consists of exposing the pieces to a silicon atmosphere at `an extremely elevated temperature in the order of 1800 C. to 2500 C. Since the siliconizing step is a major factor in the cost of production of dense silicon carbide products, it should be appreciated that an efficient apparatus for carrying out this process results in a substantial reduction in costs.

The general mode of operation of the improved furnace described above is as follows: Prior to turning the furnace on, the furnace is purged with nitrogen. Oxygen analyses are taken on the exhaust gases and when the oxygen content is reduced below 1 percent, the furnace is started and slowly heated up to its operating temperatures. The products to be furnaced and the silicon metal required for the sili-conizing step are loaded into work containing receptacles or capsules which are pushed 4by any suitable means (not shown), independent of the pusher unit through the liner tube 24 one behind the other at a con trolled continuous rate. The capsules are advanced through the preheat zone, which is maintained at approximately 600 F.-1500 F. by heating elements 98, to gradually heat the ware end and to remove any volatiles introduced by binders that might `remain in the ware. Then the capsules are advanced through the hot zone whereat the ware is iired at temperatures of 1800 C. to 2500 C. to effect the siliconizing operation. Thereafter, the capsules are pushed through the controlled cooling zone, wherein the cooling profile of the ware is carefully controlled and regulated to preclude damage that may result from thermal shock. Finally, the ware is advanced through the cooling zone and water cooled exit chamber where the temperature of the ware and capsules is further reduced to approximately room temperature for handling purposes.

Intermittently, 4while advance of the capsules is temporarily terminated, the liner tube 24 is advanced by the pusher unit 10 a predetermined increment so that individual liner tube sections 26 may be ultimately cycled through the furnace and replaced. By way of example, and for illustrative purposes only, assume that each liner tube section 26 has a length of 72 inches and that the entire composite liner tube 24 is advanced 72 inches within a 24 hour period so that each day a clean section is added at the forward end of the furnace while a used section is removed from the exit end. To preclude the liner tube sections from adhering to the graphite liner tube support slabs and other elements which they may engage as a result of silicon metal leakage, the liner tube is advanced intermittently during the 24 hour period rather than the full increment 4at any one time. If desired, at the end of each eight hour shift, the rearmost short liner tube section 27 may be removed and the liner tube pushed a distance equal to the length of such section. When the most remote of the liner tube sections 26 abuts door 222, it is removed and replaced with the shorter sections. It should be appreciated that the principles of this invention contemplate the use of sections 27 shorter than those illustrated accompanied by more frequent liner tube pushes, if desired, the important factor being that the tube be advanced relative to the stationary elements often enough to prevent adhesion therebetween.

Although the above invention was described preferably in connection with an electric resistance furnace, it should be realized that the principles of this invention envisage the application of this invention in any type of a furnace, such as an induction furnace for example. Also, the cross section of the liner tube need not be limited to a circular configuration, but may be any configuration compatible with the interior design of the furnace. The liner tubes may be moved either simultaneously with the capsules or intermittently within the purview of this invention.

It should be realized that the apparatus of this invention is in no manner restricted to the arrangement or sequence fof zones described, but rather may be incorporated in any combination of zones resulting in the temperature profile desired.

As a result of this invention, a new and improved continuous horizontal tube furnace is provided for tiring ware in an improved and more eiiicient manner resulting in increased furnace usage and appreciable reductions in costs. Another advantage residing in the furnace of the present invention is that the furnace does not have to be torn down for liner tube replacement thereby obviating time consuming and expensive down time. Not only does the furnace of the present invention solve the problem of residue buildup but also obviates cool down for removing contaminated liner tubes when it is desired to employ other tiring processes.

A preferred embodiment of this invention having been described and illustrated, it is to be realized that modifications thereof may be made without departing from the broad spirit and scope of this invention as defined in the appended claims.

We claim:

.1. An electric furnace comprising: a base; a casing mounted on said base and having an opening extending therethrough; an elongated hollow liner tube formed of a series of open ended sections adapted to encase work containing receptacles; srneans, independent of 'any means for moving said receptacles, for advancing said liner tume throu-gh said opening; and means for supporting said liner tube as it is advanced through said furnace.

2. A furnace as recited in claim 1 in which said hollow liner tube extends completely through said furnace.

3. A furnace as recited in claim 2 in which adjacent sections of said liner tube are provided with means, respectively, for detachably joining said sections together.

4. A furnace `as recited in claim 3 including a hot zone portion and at least one protection tube mounted in said hot zone portion and disposed about a portion of said liner tube,

5. A furnace as recited in claim 4 including a particulate mass of thermal insulating material surrounding said protection tube in said hot zone portion.

6. A furnace as recited in claim 5 in which said liner turbe is formed of graphite and is circular in cross section.

7. A furnace as recited in claim 6 in which said means for advancing said liner tube comprises a pusher block engageable with the liner tube and a drive means operatively connected to said pusher block for moving the same.

8. A method of replacing liner tubes in an electric furnace havin-g an entrance end and an exit end comprising: providing `a plurality of hollow open ended members; connecting said members together to form a composite liner tube adapted to encase continuously moving work containing receptacles; moving said receptacles through said furnace; advancing said composite liner tube through said furnace during ya tiring operation independently of said receptacles; removing a. remote one of said open ended members `from said liner tube at said exit end of said furnace and adding an open ended member onto said liner tube at said entrance end of said furnace without interfering with said tiring operation.

9. A method of replacing liner tubes as recited in claim 8 in which the liner tube is advanced intermittently.

10. A method of replacing liner tubes as recited in claim 8 in which the liner tube is advanced continuously at the same rate of speed as .the work containing receptacles.

11. A method ot' replacing liner tubes as recited in claim 8 in which the liner tube is advanced continuously at a rate of speed varying from the rate of speed of the work containing receptacles.

References Cited UNITED STATES PATENTS 2,125,588 8/1938 Ridgeway 13-25 2,729,542 1/ 1956 Van Der Pyl 13--26I XR 3,336,431 8/1967 Biddulph 13-25 FOREIGN PATENTS 688,998 3/ 1953 Great Britain.

ROBERT K. SCHAEFER, Primary Examiner. M. GINSBURG, Assistant Examiner.

U.S. Cl. X.R. 13-35; 2.60-276; 263-41 

